Re: Question about motor lenght on indoor high performance models

From: Tapio Linkosalo <tapio.linkosalo_at_iki.fi>
Date: Tue, 22 Aug 2017 16:54:44 +0300

On 22/8/2017 10:22, Manuel Cisneros macs8953_at_yahoo.com
[Indoor_Construction] wrote:
> I know these classes are different WRT to motor weight specification -
> F1D and F1M specify maximum motor weight, F1L doesn't - so I would
> expect the selection of a relative motor length would be different for
> different classes, but I am trying to understand the adage that seems
> to implore flyers to adjust motor dimensions based on how many turns
> are left in a motor at touchdown to optimize flight time based on
> available motor energy for different cross-section/l! ength combinations.
>
> One reason I ask is that in perusing some (admittedly older) designs,
> I've found motor lengths to be specified anywhere from 80-90% of the
> hook-to-hook distance to as long as 120%-130%.
>

Two things to consider: if the hook-to-hook distance is much shorter
than the motor, then you are likely to get motor bunching, which is not
good. Also some weight shifting depending on how to motor unwinds. On
the other hand, if the stick is much longer than the motor, you cannot
pack as many turns into the motor (which you find out if while winding
you keep the motor stretched too long - it breaks). So for (flying
classes with limited motor weight) is seems best to have the
hook-to-hook distance about the same as the motor length. Then again,
the variation you mention is still within acceptable limits.

Another thing is the impact of the motor to the location of the CG. Even
in the limited classes the motor still contributes a considerable amount
to the total flying mass of the model (in F1M it is 33%, and even in
F1D  it is 22%). So adjusting the stick length changes the location of
the motor CG, and thus the whole model CG. Especially in F1D people tend
to use longer hook-to-hook -distances to pull the CG back. Or they use a
spacer between the motor and prop for the same reason.






-Tapio-
Received on Tue Aug 22 2017 - 14:27:11 CEST